Enable specify second level domain

[sakihet]

thank you for awesome gem.
i'd like to use backlog_kit on *.backlogtool.com spaces.

what do you think?

example:

client = BacklogKit::Client.new(
  space_id: '***',
  second_level_domain: 'backlogtool',
  top_level_domain: 'com',
)

[codecov-io]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master     #2   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          24     24           
  Lines         540    541    +1     
=====================================
+ Hits          540    541    +1

Impacted Files	Coverage Δ
lib/backlog_kit/client.rb	100% <100%> (ø)	⬆️

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[emsk]

@sakihet I think it's a useful API for developers who use backlogtool.com spaces.
Thank you for your PR 😄